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cell physiology
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Lecture notes regarding cell physiology
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Cell Physiology• Calcium wave across the egg during fusion of egg and sperm, calcium in muscle
contraction (intracellular calcium store release), intracellular Ca2+ concentration
must be low so that it can be used as a signalling molecule, positive ion moving in,
causing an inward current.
• Cardiac action potential- with calcium ions (higher than potassium and chloride,
positive value of ECa)
• More slowly hence Na channels open first and conduct less current since Ca2+
carries a lot of water with it (shell of hydration)- radius of ion and viscosity of the
medium, hence if the viscosity of the medium is the same, the radius of the ion is
dependent- in this case the difference in size for calcium vs. sodium
• Differences in rates and conductance (highest in L HVA ions), also characterised by
divalent ions that can block them- understand that there is much more structural
and divergent diversity in calcium channels (unlike voltage gated potassium and
sodium channels) despite there being a near identical molecular structure to voltage
gated sodium channels- they both have the inactivation loops- energy is conserved
• All voltage gated channels evolved/ diverged from a single common ancestor so
monophyletic group, but together they are also a large polyphyletic group, it is
believed that the ancestral voltage gated cation channel was most likely modern
voltage gated calcium channels- high levels of calcium carbonate deposition found in
invertebrate lineages.
• Not all channels inactivate, but calcium channels do inactivate, using it as a signalling
molecule means that eventually the signal needs to be stopped hence these
molecules have a calcium sensor to use calcium until it is used appropriately, can be
pumped into a cell or into an organelle (sarcoplasmic/ ER)
• Calcium dependent proteins inside the cell, calcium acts as an activation cascade
(intracellular signalling molecule) affecting contraction, secretion, channel gating,
activating multiple enzymes and influences gene expression. Single signal so multiple
pathways can be activated at the same time
• Other examples of intracellular signalling molecules include nitrous oxide (NO),
produced by cells, cAMP and cGAMP are also signalling molecules
pH
• Extremely alkaline pH in mosquitos and caterpillars- tannins, presents an interesting
way to control insects (BTI is a toxic and thrown into water basin and eaten by
mosquitos- the gut causes it to become active; same for BTK for caterpillars)
Intracellular pH regulation
• Use of Nernst equation to calculate pH of cell- 6.21 pH, therefore pH is a steady state
equilibrium
• Optimal pH of enzymes/ proteins typically at 7 inside cells, hence this needs to be
maintained steady state pH- cells are under a constant acid load (products of
oxidative metabolism are protons from oxygen and water)
• Protons tend to leak in whilst OH- and HCO3- ions tend to leak out hence 3- cells are
under a constant acid load, the rate of acid loading must be equivalent to the rate of
acid extrusion
, • Proteins can leak through most channels when these are opening hence also part of
the acid load, mostly in cation channels (channels are based on charge and size),
driving force is in. Vacuoles and lysosymes (acidic intracellular organelles) will leak
from intracellular compartments into the cytosol. In some tissues, the proton
gradient is used to drive other processes, like the uptake of ions (active influx and
efflux reactions)
• Generation and consumption of protons, e.g. glycolysis has the production of
proton. Balance in process of sequestering H+ ions. Consumption of carbohydrates.
Creatine phosphate is used as a fuel source once all sugar has been consumed (fatty
acids are better fuels), but also reducing acid load on cells- but in theory, this won’t
be in the system, the majority leaves the system- respiration rate isn’t increased.
During exercise, build-up of lactic acid, and breathing more- CO2 + 02, hence
increase in acid load (Hasselhoff equation)
• The intracellular pH is not constant- with increases in body temperature, the blood
pH level (extracellular?) decrease
• Decreasing temp, increase in buffering groups- but the proton concentration
decreases and charge on protein remains constant to maintain protein activity as
this is means protein is at optimal activity. Protein becomes less active with acid
load. Biological processes need to still occur regardless of the change in body
temperatures. Hence must expend energy to maintain steady state equilibrium.
• The intracellular pH can be measured using homogenates but results in
disequilibrium (won’t reflect the pH, underestimating driving force of protons),
acidification of subcellular organelles- the pH would be more overestimated, more
alkaline than it should be. It can also be measured using microelectrodes- use of pH
meters- prefabricated chemicals ‘cocktail’, looking for a change in voltage (change in
protons plus membrane potential), and change in concentration. Reference meter to
measure membrane potential- measured inside relative to outside. Fluorescent Dyes
can also be used- use of fluorescein (emits a florescent light), this compound outside
cells can load the cell with this dye, eventually leaves BCECF trapped so it can’t leak
out and this pH can be quantified since it binds to so many protons. Leads to
spectrums, difference in ratio at every intracellular pH.
• pH levels usually range from 6.5 to 8.5, so this is a good tool for measuring pH levels
• pH mechanism takes advantage of the fact that the sodium is made functionally
impermeable, the sodium goes from out to in with protein and pumped out using
sodium pump, Em isn’t changed- electroneutral, no change in potential. Driving force
for potassium is typically out for cells, hence can acidify cells and is again
electroneutral. Also, anion and base transporters, chloride-bicarbonate exchange-
main pH regulatory mechanisms used by all cells, where Cl goes in and bichloride
goes out and removing a base equivalent. Sodium gradient advantage and removing
2 acid equivalents- none of these have affected membrane potential. The proton
pumps- P-type and V-type. V-type typically in vacuole, using ATP, typically in
vertebrates, used by invertebrates to act as an acid excruder, in both case this
causes the charge in the vacuole to become more positive, cytosol is more negative
relative to outside, hence driving force for sodium efflux is increased- doesn’t
diminish the sodium gradient for other processes. P-type- H-K transporter typically
makes the outside of the cell acidic in the gastric epithelium at very high
concentrations to maintain a very acid pH in the stomach, this pump is blocked to
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